Generally, an active sonar emits sound waves into the seawater and receives the sound waves reflected by the target object. The position of the target object and the distance to the target object are measured by using the time difference between sound emission and reception and the incident direction of the reflected sound waves. However, the received sound includes noise generated by waves or the like besides the reflected sound. Accordingly, it is difficult to detect the target object with a high degree of accuracy. Various ideas for improving the detection accuracy are offered.
For example, a method by which even a broadband signal can be detected is disclosed in Japanese Patent Application Laid-Open No. 2004-118108. In this method, by performing an FFT process of acoustic signal data, a time integration is performed for each frequency and a local average value of a section length of a predetermined frequency is obtained. An amplitude level of the frequency is divided by the local average value to perform local normalization. After this, a frequency width that is set in advance is used as an average section length and the local average value is obtained. By replacing the amplitude level value of the frequency with the local average value, the noise is eliminated.
In Japanese Patent Application Laid-Open No. 2005-351721, a method by which by compressing beam data obtained by performing directivity synthesis for each direction into an amount of one channel, a signal is displayed without losing a process gain of the directivity synthesis process is disclosed. In this method, a plurality of directive beams are formed, a frequency analysis is performed, an average level of the noise is calculated, and the normalization is performed. Additionally, the normalized signal is integrated to suppress the noise variation.
Further, a method for detecting a carrier with high accuracy is disclosed in Japanese Patent Application Laid-Open No. 2009-253911. In this method, an input signal is Fourier transformed, a power operation is performed over an integration period, and a normalized autocorrelation value in which noise is suppressed is calculated. It is determined whether or not a narrowband signal is included in a desired frequency bin according to whether or not the normalized autocorrelation value is greater than a threshold value.
However, it is difficult to sufficiently suppress the noise by the method described in each patent document mentioned above. For example, in the method disclosed in Japanese Patent Application Laid-Open No. 2004-118108, the time integration is performed for each frequency by performing the FFT process on the acoustic signal data. However, because the integration time constant used in the integration is fixed, there is a case in which it does not correspond to the frequency of the noise. Namely, when it is considered that a sound source of the noise is a wave, there are two types of waves: a deep-surging wave and a breaking wave. Namely, the frequency of the wave is not a single frequency. Accordingly, even when the time integration is performed for each frequency, the noise cannot be sufficiently suppressed when the integration time constant is fixed.